Conventionally used refrigerants for refrigerators include a non-azeotropic mixture (R407D) of difluoromethane (R32)/pentafluoroethane (R125)/1,1,1,2-tetrafluoroethane (R134a) (15/15/70% by mass) and a non-azeotropic mixture (R404A) of pentafluoroethane (R125)/1,1,1-trifluoroethane (R143a)/1,1,1,2-tetrafluoroethane (R134a) (44/52/4% by mass). The boiling point of R407D is about −39° C., and the boiling point of R404A is about −46° C., and these are suitable for known refrigerating apparatuses. Furthermore, even when a compressor inlet temperature is relatively high, an outlet temperature does not increase to a degree that oil sludge is caused in a compressor. However, R404A has a relatively high GWP of 3920.
On the other hand, for obtaining a lower temperature range of −80° C. or lower, R508A (an azeotropic mixture of trifluoromethane R23 and hexafluoroethane R116) is used. The boiling point of R508A is −85.7° C., and suitable for obtaining low temperature.
However, the above-described refrigerants each have a physical property that the global-warming potential (the GWP value) is extraordinarily high.
This refrigerant composition is either a refrigerant composition including an azeotropic mixture (R508A, boiling point: −85.7° C.) of 39% by weight of trifluoromethane (R23) having a rather high specific heat ratio and 61% by weight of hexafluoroethane (R116) having a rather low specific heat ratio, or a refrigerant composition including a mixture of this azeotropic mixture and n-pentane or propane that is mixed in a ratio of 14% or less with respect to a total weight of trifluoromethane and hexafluoroethane. This refrigerant composition can achieve a low temperature of −80° C.
However, the above-described R508A has a high GWP of 13200, which has been a problem.
Carbon dioxide (R744) has a small GWP of 1, but has a problem that oil deterioration and sludge occur due to increase in pressure and outlet temperature. Therefore, there are proposed: a refrigerant mixture in which carbohydrates such as propane, cyclopropane, isobutene, and butane are added to carbon dioxide in a ratio of about 30 to 70% of the whole; and a refrigeration cycle apparatus including the refrigerant mixture (see Patent Literature 1).
Furthermore, there are proposed: a refrigerant mixture containing isobutane in a ratio of 40 to 60% and trifluoromethane (R23) as the remainder (see Patent Literature 2); a refrigerant mixture in which propane is mixed in a ratio of 65% or more to a mixture of difluoromethane and pentafluoroethane (see Patent Literature 3); and the like.